Seismic surveying is used for identifying subterranean elements, such as hydrocarbon layers, fresh water aquifers, and so forth. In performing seismic surveying, seismic sources are placed at various locations on an earth surface or sea floor (or in a wellbore), with the seismic sources activated to generate seismic waves directed into a subterranean structure. Examples of seismic sources include explosives, air guns, or other sources that generate seismic (acoustic) waves. In a marine environment, seismic sources and sensors can be towed in water by a sea vessel.
The seismic waves generated by a seismic source travel into the subterranean structure, with a portion of the seismic waves reflected back to the surface (earth surface, sea floor, or wellbore surface) for receipt by seismic sensors (e.g., geophones). These seismic sensors produce signals that represent detected seismic waves. Signals from the seismic sensors are processed to yield information about the content and characteristic of the subterranean structure.
Recorded seismic data is often degraded by the presence of internal multiples (IMs) in the recorded seismic data. Internal multiples in the seismic data are caused by the presence of one or more internal multiple (IM) generators between the surface (earth surface or sea floor,) and a target reflector (such as a hydrocarbon layer, fresh water aquifer, and so forth). An IM generator is caused by changes in the density or velocity of the subterranean structure. The presence of an IM generator between the recording surface and a reflector causes multiple reflections to occur between the IM generator and the reflector. Thus, for example, a seismic wave that travels downwardly into the subterranean structure will have a portion that is reflected back from the IM generator, and have another portion that passes through the IM generator to a reflector. A seismic wave is then reflected from the reflector back up towards a recording surface (where seismic sensors are located). A portion of this reflected seismic wave travels through the IM generator to the recording surface. However, another portion of this reflected seismic wave is reflected back downwardly by the IM generator towards the reflector, which is then followed by further reflection from the reflector up towards the recording surface. Such reflections between the IM generator and reflector can occur multiple times. Seismic data due to the reflections between the IM generator and the reflector are referred to as internal multiples. The presence of internal multiples in the recorded seismic data pollutes the recorded seismic data and leads to decreased accuracy in surveying a subterranean structure.